1. Field of the Invention
Generally, the present disclosure relates to the field of fabricating products, such as semiconductor devices, in a manufacturing environment including process tools exchanging transport carriers with an automated transport system, wherein the products, such as substrates for semiconductor devices, are processed on the basis of groups or lots defined by the contents of the transport carriers.
2. Description of the Related Art
Today's global market forces manufacturers of mass products to offer high quality devices at a low price. It is thus important to improve yield and process efficiency to minimize production costs. This holds especially true in the field of semiconductor fabrication, since, here, it is essential to combine cutting-edge technology with volume production techniques. It is, therefore, the goal of semiconductor manufacturers to reduce the consumption of raw materials and consumables while at the same time improve process tool utilization. The latter aspect is especially important since, in modern semiconductor facilities, equipment is required which is extremely cost intensive and represents the dominant part of the total production costs.
Integrated circuits are typically manufactured in automated or semi-automated facilitics, by passing substrates comprising the devices through a large number of process and metrology steps to complete the devices. The number and the type of process steps and metrology steps a semiconductor device has to go through depends on the specifics of the semiconductor device to be fabricated. A usual process flow for an integrated circuit may include a plurality of photolithography steps to image a circuit pattern for a specific device layer into a resist layer, which is subsequently patterned to form a resist mask for further processes in structuring the device layer under consideration by, for example, etch or implant processes and the like. Thus, layer after layer, a plurality of process steps are performed based on a specific lithographic mask set for the various layers of the specified device. For instance, a sophisticated CPU requires several hundred process steps, each of which has to be carried out within specified process margins to fulfill the specifications for the device under consideration. Since many of these processes are very critical, such as many photolithography steps, a plurality of metrology steps have to be performed to efficiently control the process flow and to monitor the performance of the respective process tools. For example, so-called pilot substrates are frequently processed and subjected to measurement procedures prior to actually releasing the associated group of “parent” substrates in order to test the compliance with predefined process margins. Typical metrology processes may include the measurement of layer thickness, the determination of dimensions of critical features, such as the gate length of transistors, the measurement of dopant profiles and the like. As the majority of the process margins are device specific, many of the metrology processes and the actual manufacturing processes are specifically designed for the device under consideration and require specific parameter settings at the adequate metrology and process tools.
In a semiconductor facility, a plurality of different product types are usually manufactured at the same time, such as memory chips of different design and storage capacity, CPUs of different design and operating speed and the like, wherein the number of different product types may even reach a hundred and more in production lines for manufacturing ASICs (application specific ICs). Since each of the different product types may require a specific process flow, different mask sets for the lithography, specific settings in the various process tools, such as deposition tools, etch tools, implantation tools, chemical mechanical polishing (CMP) tools and the like, may be necessary. Consequently, a plurality of different tool parameter settings and product types may be encountered simultaneously in a manufacturing environment. Thus, a mixture of product types, such as test and development products, pilot products, different versions of products, at different manufacturing stages may be present in the manufacturing environment at a time, wherein the composition of the mixture may vary over time depending on economic constraints and the like, since the dispatching of non-processed substrates into the manufacturing environment may depend on various factors, such as the ordering of specific products, a variable degree of research and development efforts and the like. Thus, frequently, the various product types may have to be processed with a different priority to meet requirements imposed by specific economic or other constraints.
Despite these complex conditions, it is an important aspect with respect to productivity to coordinate the process flow within the manufacturing environment in such a way that high performance, for example, in terms of tool utilization, of the process tools is achieved, since the investment costs and the moderately low “life span” of process tools, particularly in a semiconductor facility, significantly determine the price of the final semiconductor devices. In modern semiconductor facilities, a high degree of automation is typically encountered, wherein the transport of substrates from and to the process and metrology tools is accomplished on the basis of respective transport carriers accommodating a specific maximum number of substrates. The number of substrates contained in a carrier is also referred to as a lot and the number of substrates is therefore frequently called the lot size. In a highly automated process line of a semiconductor facility, the transport of the carriers is mainly performed by an automated transport system that picks up a carrier at a specific location, for example, a load port associated with a process or metrology tool, within the environment and delivers the carrier to its destination, for instance, a load port of another process or metrology tool that may perform the next process or processes required in the respective process flow of the products under consideration. Thus, the products in one carrier typically represent substrates to be processed in the same process tool, wherein the number of substrates in the carrier may not necessarily correspond to the maximum number of possible substrates. That is, the lot size of the various carriers may vary, wherein typically a “standard” lot size may dominate in the manufacturing environment. For example, one or more pilot substrates, which may be considered as representatives of a certain number of parent substrates contained in a certain number of carriers filled with the standard lot size, may be transported in a separate carrier, since they may undergo a specific measurement process and therefore may have to be conveyed to a corresponding metrology tool, thereby requiring an additional transport job. Based on the results of the measurement process, the waiting parent substrates may then be delivered to the respective process tool.
The supply of carriers to and from process tools is usually accomplished on the basis of respective “interfaces,” also referred to as loading stations or load ports, which may receive the carriers from the transport system and hold the carriers to be picked up by the transport system. The transport system comprises a rail system that is typically attached to the clean room ceiling so that the transport system is usually referred to as an overhead transport system (OHT). Furthermore, the OHT accommodates a plurality of vehicles running along the OHT rails in order to convey a transport carrier that is to be exchanged with a specific process tool by means of one or more load ports associated with the tool under consideration. Due to the increasing complexity of process tools, having implemented therein a plurality of functions, the cycle time for a single substrate may increase. Hence, when substrates are not available at the tool although being in a productive state, significant idle times or unproductive times may be created, thereby significantly reducing the utilization of the tool. Thus, typically, the number and configuration of the load ports is selected such that one or more carriers may be exchanged at the load port(s) while the functional module of the process tool receives substrates from another load port to achieve a cascaded or continuous operation of the functional module of the process tool. The time for the exchange of carriers between the automated transport system and the respective process or metrology tool depends on the transport capacity of the transport system and the availability of the carrier to be conveyed at its source location. Ideally, when a corresponding transport request for a specified lot currently processed in a source tool is to be served, the respective substrates should be available at the time the transport system picks up the carrier including the lot and delivers the carrier at the destination tool such that a continuous operation may be maintained. Consequently, the respective carrier should be delivered to the destination tool when or before the last substrate of the carrier currently processed in the destination tool is entered into the process module so that a continuous operation may be achieved on the basis of the newly arrived carrier. Thus, for an ideal continuous operation of a process tool, one carrier would be exchanged while another carrier is currently processed.
Depending on the capacity of the tool interface, for instance, the number of load ports provided, a certain buffer of carriers and thus substrates may be provided in order to generate a certain tolerance for delays and irregular deliveries, which may, however, significantly contribute to tool costs. In some circumstances, the required carrier exchange time for maintaining a continuous operation of the tool under consideration may even be negative, thereby requiring a change of the substrate handling scenario. Moreover, the actual carrier exchange time, i.e., the time required for picking up a full carrier including processed substrates from the load port and putting a carrier onto the load port to provide new substrates to be processed, does not substantially depend on the lot size, whereas the time window for the opportunity to perform an actual carrier exchange is highly dependent on the respective lot size, since a small currently processed lot provides only a reduced time interval for exchanging another carrier without producing an undesired idle time, also referred to as a window of opportunity for carrier exchange. Thus, the presence of a mixture of lot sizes, such as pilot lots, development lots and the like, or the presence of lots having a high priority, may negatively affect the overall performance of process tools.
Moreover, in view of cycle time enhancement for the individual products and to address flexibility in coping with customers' specific demands, the lot size may decrease in future process strategies. For example, currently 25 wafers per transport carrier may be a frequently used lot size, wherein, however, many lots may have to be handled with a lesser number of wafers due to the above requirements, thereby imposing a high burden on the process capabilities of the automatic transport system and the scheduling regime in the facility in order to maintain a high overall tool utilization. That is, the variability of the carrier exchange times for exchanging the carriers with respective load stations of the process tools may be high and thus a significant influence of the transport status in the manufacturing environment on the overall productivity may be observed. Thus, when designing or re-designing a manufacturing environment, for instance, by installing new or additional equipment, the tool characteristics with respect to transport capabilities, such as the number of load ports for specific tools and the like, and the capabilities and operational behavior of the automatic material handling system (AMHS) may represent important factors for the performance of the manufacturing environment as a whole. The handling of small and different lot sizes within the manufacturing environment that is designed for a moderately large standard lot size may, therefore, require highly sophisticated scheduling regimes to compensate for the lack of sufficient carrier exchange capacity in the existing tools. However, the presence of small lot sizes may nevertheless result in a significant reduction of tool utilization, in particular in photolithography tools and related process tools, which are responsible for an essential part of the total production costs, as previously explained, due to the fact that a negative carrier exchange time that may be associated with the processing of small lot sizes may not be compensated for, unless significant modifications of the process tools under consideration are made in view of increasing the I/O capabilities of the tool. That is, the number of load ports may have to be increased, as will be described in more detail with reference to FIGS. 1a-1f. 
FIG. 1a schematically illustrates a cross-sectional view of a manufacturing environment 150 representing a portion of a semiconductor facility for manufacturing microstructure features, such as integrated circuits and the like. The manufacturing environment 150 is typically established within a clean room, in which the environmental conditions, such as temperature, humidity, the number of airborne particles and the like, are controlled to be within tightly set ranges. Consequently, the floor space within a respective clean room is very expensive, wherein the productivity per unit area of the clean room may contribute significantly to the overall production costs of semiconductor devices. The manufacturing environment 150 comprises a plurality of process tools, such as lithography tools, etch tools, deposition tools, anneal tools and the like, wherein, for convenience, a single process tool 160 is illustrated in FIG. 1a, which may represent a lithography tool, possibly in combination with any related process tools required for performing sophisticated lithography processes as are typically necessary for the formation of advanced semiconductor devices. For example, the process tool 160 may represent one or more exposure tools in combination with other process modules required for depositing a photoresist, baking the resist, developing the resist and the like. As previously explained, lithography processes may represent one of the most cost-intensive process steps, in particular when highly critical exposure steps are considered, such as the patterning of gate electrodes and the like. The process tool 160 may be associated with a carrier exchange interface 161, which may be provided in the form of a plurality of load ports that are configured to receive substrate carriers including substrates to be processed and to feed the substrates contained therein into the process modules included in the process tool 160. Furthermore, the manufacturing environment 150 comprises an overhead transport (OHT) system 170, which typically comprises appropriate transport rails 171 configured to accommodate and guide transport vehicles 172 which in turn are configured to receive substrate carriers 173 that are to be exchanged with the interface 161. For this purpose, typically, the transport vehicles 172 are appropriately configured to pick up a respective substrate carrier 173 from a source load port of a process tool within the manufacturing environment 150 and convey the carrier 173 on the basis of the rails 171 to a destination within the environment 150, such as the process tool 160. For this purpose, the rails 171 are typically attached to the ceiling 174 of the respective clean room such that the vehicles 172 containing the carrier 173 under consideration may be positioned vertically above a respective one of the load ports of the interface 161. Based on an appropriately designed mechanism (not shown), the carrier 173 may be hoisted down so as to be positioned on the respective load port.
FIG. 1b schematically illustrates a top view of the manufacturing environment 150 as shown in FIG. 1a wherein, by way of example, the process tool 160 is connected to the interface 161 having five load ports indicated as L1, L2, L3, L4, L5. As shown, one of the rails 171 may be positioned such that the corresponding transport vehicles 172 moving in the downward direction of FIG. 1b may be positioned above a respective one of the load ports L1, L2, L3, L4, L5, depending on whether any of the load ports L1, L2, L3, L4, L5 is available for receiving the substrate carrier 173 or has a corresponding substrate carrier 173 that has to be picked up by the vehicle 172.
During operation of the manufacturing environment 150, a supervising control system, such as an MES (manufacturing execution system) (not shown), typically provided in complex manufacturing environments, may identify a group of substrates, also referred to as a lot, that has to be processed in the process tool 160. In this case, the group of substrates may be positioned in one of the carriers 173 and this carrier 173 may be picked up by the transport system 170 from a corresponding load port of a source process tool or any other location within the manufacturing environment 150. Upon arriving at the process tool 160, an available load port may be identified and the respective carrier 173 may be positioned at this load port and may then be unloaded to supply the substrates to the tool internal process modules for performing one or more processes required by the specific process flow for the substrates under consideration. As previously mentioned, in modern semiconductor facilities, not only the quality of respective processes has to be monitored and maintained within tight process margins, but also the throughput of the process tool 160 is an important factor in view of overall production costs. Therefore, it is an important aspect in managing the complex manufacturing environment 150 to supply substrates to the tool internal modules in a substantially continuous manner to substantially avoid idle times of the process modules of the tool 160. Consequently, the scheduling of the arrival of substrate carriers 173 is typically performed in such a manner that the substrate carriers 173 arrive at the various load ports L1, L2, L3, L4, L5 without resulting in undue idle times of the process tool 160. For example, the lot size in the manufacturing environment 160 may typically be 25 substrates per carrier 173 and the number of load ports of the interface 161 is typically selected so as to allow the arrival of a sufficient number of substrate carriers 173 in order to obtain a substantially continuous operation of the process tool 160.
By way of example, the process tool 160 may represent a lithography tool or a respective tool cluster designed for a throughput of approximately 120 substrates per hour, wherein the tool internal process modules may receive 70 substrates until the first substrate is output back to the carrier exchange interface 161. In a typical process regime, a substrate carrier 173 is received by one of the load ports L1, L2, L3, L4, L5, is unloaded and waits until the substrates are processed by the tool 160 and are returned to the same substrate carrier 173. That is, the substrate carriers 173 used for transport of substrates to respective load ports of the interface 161 have to stay attached to the load ports, while the substrates are being processed in the tool 160. Under these conditions, the continuous processing in the process tool 160 may be obtained by providing an appropriate number of load ports in the interface 161, thereby ensuring that a sufficient number of substrates are present in the process tool 160 at any time. However, there is a general tendency for reducing the number of substrates per lot, for instance using 12 substrates per lot instead of 25, in order to reduce the overall process time for a single substrate. In future strategies for operating semiconductor facilities, even smaller lot sizes have been proposed wherein, in view of flexibility and reduction of overall process time, lot sizes as small as one substrate may be used, in particular if the size of the individual substrates is increased. Thus, when reducing the lot size several, process tools may run into throughput problems due to a non-continuous operation of the process tool, since the existing number of load ports may not allow a continuous operation.
FIG. 1c schematically illustrates a timing diagram for the operation of the process tool 160 under the above-specified conditions, i.e., a throughput of 120 substrates per hour with 70 substrates being simultaneously processed within the tool internal process modules of the tool 160.
In FIG. 1c, load port LP1 may have received a substrate carrier 173, the substrates of which may immediately be loaded into the tool 160, wherein it may be assumed that the respective transport activities in the load ports LP1, LP2, LP3, LP4, LP5, performed by appropriately designed substrate handling systems (not shown), may take 12.5 minutes for unloading 25 substrates. Thus, after the time interval t1, 25 substrates are in the tool internal modules of the tool 160. Thereafter the substrate carrier remains empty on the load port LP1, representing an idle time interval t2 of the load port LP1. Since the other load ports LP2, LP3, LP4, LP5 may have also received or may still receive respective substrate carriers 173, thereby further feeding the tool 160 for maintaining a continuous operation, the idle time t2 may be 22.5 minutes for the load port LP1 until receiving a substrate back from the tool 160. It may, for instance, be assumed, as shown in FIG. 1c, that the load ports LP1, LP2, LP3, LP4, LP5 are sequentially served and hence, after a respective time interval t1 corresponding to the load port LP2, 50 substrates are within the tool 160 when a restart situation with a previously empty tool 160 is considered, thereby requiring another 10 minutes of the time interval t1 associated with the load port LP3, until 70 substrates are in the tool 160. Thereafter, during the time interval t3, which may also take 12.5 minutes, the substrates are sequentially returned to LP1, while the remaining substrates of the carrier on LP3 and further substrates of the carrier on load port LP4 are continuously supplied to the tool 160. Thus, after 10 minutes of time interval t1 corresponding to LP4, the carrier at the load port LP1 is refilled and is ready for being picked up by a vehicle 172 of the transport system 170. In the next 2.5 minutes, the remaining substrates of the carrier at load port LP4 are supplied and thereafter substrates of a carrier positioned on load port LP5 are supplied to the tool 160, thereby providing a time interval t4 of 15 minutes for preparing the carrier for being picked up, positioning a new carrier at the load port LP1 and preparing the newly supplied carrier for supplying substrates to the tool 160. For instance, it may be assumed that corresponding activities, such as closing the substrate carrier, opening the substrate carrier and the like, may take approximately 0.5 minutes, so that a time period of 14 minutes may remain for a carrier exchange with the transport system 170. A corresponding time window is typically within the transport capabilities of the system 170. Hence, for maintaining a substantially continuous operation on the basis of a standard lot size of 25 substrates per carrier, the provision of five load ports for the interface 161 seems to be appropriate.
FIG. 1d schematically illustrates a timing diagram for a situation in which the interface 161 comprises only four load ports LP1, LP2, LP3, LP4 for otherwise identical conditions. Since substrates have to be supplied to the tool 160 by a new carrier of LP1 after the carrier of load port LP4 has been emptied, the time interval t4 in FIG. 1d is reduced by 12.5 minutes, thereby resulting in a time window of 1.5 minutes as the respective carrier exchange time. However, a corresponding small carrier exchange time may be beyond the capabilities of the transport system 170, thereby rendering this solution as less attractive, even though significant cost savings may be obtained due to reduced floor space and investment costs for providing the interface 161 having a reduced number of load ports.
FIG. 1e schematically illustrates a timing diagram for operating the process tool 160 on the basis of a small standard lot size, for instance 12 substrates per lot, while additionally it is assumed that seven load ports LP1, LP2, LP3, LP4, LP5, LP6, LP7 may be provided for the interface 161. Other operating conditions regarding the process tool 160 may be identical to those described above with reference to FIGS. 1c-1d. As shown, the time interval t1 at load port LP1 is therefore 6 minutes, followed by the time interval t2 which lasts 29 minutes, representing a period in which a respective substrate carrier 173 is positioned at the corresponding load port waiting for the substrates to return from the tool 160. Consequently, during the time interval t1 at load port LP6, ten substrates are unloaded during the first five minutes, thereby providing the required 70 substrates to the tool 160, while the remaining two substrates from load port LP6 and the time interval t1 of the load port LP7 have to cover the time required for loading the substrates returning to load port LP1 into the respective substrate carrier. Consequently, time interval t3 is 1 minute, thereby resulting in a respective time window or carrier exchange time of 0 minutes.
FIG. 1f schematically illustrates a timing diagram for an operating situation in which the interface 161 comprises eight load ports LP1, LP2, LP3, LP4, LP5, LP6, LP7, LP8 in order to increase the respective time interval t4 to 7 minutes, thereby obtaining a carrier exchange time of 6 minutes, which may be within the capabilities of the transport system 170.
As a consequence, by reducing the standard lot size from 25 substrates, as illustrated in FIGS. 1c-1d, to 12 substrates, the number of load ports has to be increased, which would involve considerable hardware modifications in the environment 150, or which may even be not feasible for many existing manufacturing environments. Consequently, a significant loss of throughput of the process tool 160 may result since the corresponding carrier exchange time may even be negative when using less than seven load ports in the scenario illustrated with reference to FIGS. 1e-1f. The situation may become even worse if the processing of pilot substrates, engineering lots and the like are taken into consideration, which may usually have a lesser number of substrates, such as one substrate per lot and the like, since, in this case, the carrier exchange time in the “vicinity” of the processing of the pilot substrates may be influenced by the presence of corresponding lots, thereby contributing to a throughput loss of the process tool 160.
The present disclosure is directed to various methods and systems that may avoid, or at least reduce, the effects of one or more of the problems identified above.